A cartoon representation of the photoconversion of Cy5 between a fluorescent and a nonfluorescent state. In this diagram, an antibody molecule is labeled with the green fluorophore Cy3 and the red fluorophore Cy5. When Cy5 is exposed to red light, it emits fluorescence and then switches to a dark nonfluorescent state. Illumination with green light, however, causes Cy5 to revert to its fluorescent state by means of a short-range interaction with the neighboring Cy3. Under illumination conditions allowing single-molecule detection, this single-molecule fluorescent switch can be cycled on and off for hundreds of cycles before permanent photobleaching occurs.